At 21z (5 pm EDT) back on Wednesday, 09/22, Tropical Depression Eighteen was declared to have developed in the open Tropical Atlantic by the National Hurricane Center (NHC). Here we are, eight days later, and that Tropical Depression is now a powerful Category 4 Hurricane Sam. Granted, Sam has been a powerful storm for several days.
At the time of this post, Sam has maximum sustained winds of 125 kts (145 mph) and a minimum central pressure of 937 mb per the 30th/15z NHC official advisory and is located about 365 miles north-northeast of the Northern Leeward Islands of the NE'rn Caribbean.
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| GOES East IR Loop of Hurricane Sam (tropicaltidbits.com) |
Sam has undergone a few eyewall replacement cycles (ERC) that have resulted in fluctuations in intensity over the past few days, but Sam has re-strengthened over the last 24-36 hours or so. Again, thankfully, Sam is out over the open Atlantic and is not posing any immediate threat to land. So, where is Sam heading? Several days ago, forecasters, including myself, were anticipating a complex weather pattern to develop over the Eastern U.S. and W'rn Atlantic. This pattern involved a complex interaction of mid-upper-level ridging and troughing and has generally come to fruition. Remember, hurricanes and tropical cyclones, in general, are like leaves in a large stream. They will typically be directed where to go by the overall environment and steering currents. Sam is no different. It is now more clear that the upper-level trough currently responsible for cool weather and lake-effect clouds over New York State is helping to create a weakness in the large-scale W'rn Atlantic ridging; thus providing a path for Sam to move northwest, north, and eventually northeast. Thankfully, it appears this northward turn will come at a time that will allow Sam to bypass Bermuda to the east. Still, large and destructive waves/swell as well as the potential for localized squalls, will affect the island nation Friday night and Saturday morning, where a Tropical Storm Watch is currently in effect, with the swell/rip currents persisting through the weekend. The large-scale pattern can be depicted in the 500-mb height projection courtesy of Tropical Tidbits below.
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| 500-mb GFS forecast valid 01 Oct/12z (tropicaltidbits.com) |
You can see the broad ridge to the northeast of Sam in the plot above, as well as the deep trough that will be centered over Northern New England and Atlantic Canada. You can also easily make out the "weakness" in the ridging over the W'rn Atlantic with the ridge strengthening to the west. We will then see the trough move east and help to swoop Sam NE'ward as it eventually begins its extra-tropical transition (ETT) while likely staying offshore Newfoundland by the early to middle part of next week. Sam will then become a powerful and large storm-force cyclone over the North Atlantic.
So, will we see anything in terms of impacts for on the East Coast and Northeast from Sam? Yes, but nothing in terms of direct impacts (rain, wind, etc.). What we will see are large waves and rip currents at our ocean-facing beaches through the weekend and into early next week. This can be seen in the plot below of wind-driven combined seas/swell in excess of 5 ft offshore in more exposed waters. Again, please be aware of the rip current risk if you're heading to the beach to kick off October!
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| GFS Sig Height (ft) valid 03 Oct/00z (tropicaltidbits.com) |
The bottom line with Sam: the Northeast Caribbean dodged a powerful bullet. Sam is feisty storm, and will only add to the high frequency of storms this 2021 season. Thankfully, Sam does not appear to have a landfall in store in its future at this time, but will continue to pose hazards and risks to Atlantic shipping routes.
Thanks for reading and be safe out there!
~ Zack
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